Al-mg-zn alloy coated ferrous metal sheet



United States Patent 3,505,043 Al-Mg-Zn ALLOY COATED FERROUS METAL SHEET Harvie H. Lee, Park Forest, 11]., and James W. Halley, Chesterton, Ind., assignors to Inland Steel Company, Chicago, 111., a corporation of Delaware No Drawing. Continuation-in-part of application Ser. No. 623,241, Mar. 15, 1967. This application Jan. 8, 1969, Ser. No. 789,943

Int. Cl. B23p 3/10 US. Cl. 29-1965 7 Claims ABSTRACT OF THE DISCLOSURE A corrosion resistant zinc base alloy coating for a metal surface normally subject to attack by corrosion and containing as the essential alloying elements for improving corrosion resistance between about 1 and percent by weight magnesium and between about 3 and 17 percent by weight aluminum, with the remainder of the alloy coating being essentially zinc. The hot dip bath immersion and vapor deposition techniques are suitable methods of applying the improved alloy coating.

The present application is a continuation-in-part application of US. patent application Ser. No. 623,241, filed Mar. 15, 1967, and now abandoned which was a continuation-in-part of applicants U.S. application Ser. No. 483,032, filed Aug. 29, 1965 and now abandoned.

The present invention relates generally to an improved zinc metal alloy protective coating, and more particularly to a method of providing an improved magnesiumaluminum-Zinc alloy protective coating and to a ferrous metal article having a novel magnesium-aluminum-zinc base alloy protective coating.

The zinc hot dip coatings which have heretofore been used to protect ferrous metal surfaces against attack by corrosion, have contained various alloying metals for the purpose of improving the properties of the metal coating or for improving the hot dip coating process. Among the metals which have been added to a molten zinc coating bath are lead, tin, silicon, aluminum, cadmium, antimony and magnesium.

Ternary magnesium-aluminum-zinc base alloy compositions containing from 2 to 5% magnesium and 2% aluminum have heretofore been prepared and tested as cast block for corrosion resistance. While these alloy compositions were found to exhibit good corrosion resistance under the test conditions, the alloy compositions were incapable of being applied as a hot dip protective coating on a ferrous metal or like surface (see Corrosion, volume 6, pages 195-2'00, June 1950).

An improved zinc-magnesium alloy coating composition which has been found suitable for hot dip applications on a ferrous metal surface has more recently been discovered and contains between about 1.5% and 4% by weight magnesium and preferably a fraction of 1% by weight aluminum (i.e. up to 0.2% by weight aluminum) in order to reduce top skimming losses and to prevent formation of objectionable intermetallic compounds. Even when magnesium and aluminum are used at the foregoing concentrations in the magnesium-zinc coating bath, however, a small but definite intermetallic layer is formed and irregularities are present in the surface of the coating which detracted from the appearance of the coating. Thus, a definite need still remains for a protective metal alloy coating bath and coating composition having improved properties.

It is therefore an object of the present invention to provide an improved method of coating a metal base to produce a magnesium-alumin-um-zinc alloy protective coating having improved properties.

It is a further object of the present invention to provide an improved ferrous metal article having an improved zinc base alloy protective coating containing magnesium, aluminum and Zinc.

It is still another object of the present invention to provide an improved hot dip magnesium-aluminum-zinc alloy protective coating and coating bath for a ferrous metal strip.

It is also an object of the present invention to provide a hot dip magnesium-aluminum-zinc base alloy protective coating having improved corrosion resistance when exposed to a salt containing medium.

It is still another object of the present invention to provide a hot dip zinc base alloy coating containing a high level of magnesium and aluminum which exhibits good adherence to a ferrous metal base.

It is still another object of the present invention to provide a hot dip zinc base alloy coating containing a high level of magnesium and aluminum which has a smooth surface appearance and uniform coating thickness.

It is a further object of the present invention to provide a hot dip zinc base alloy coating containing a high level of magnesium and aluminum which exhibits improved corrosion resistance on exposure to a highly humid atmosphere.

Other objects of this invention will be apparent to those skilled in the art from the detailed description and claims to follow.

It has now been discovered that a protective metal alloy coating having markedly improved properties can be provided on a metal article, such as a ferrous metal strip, by applying thereto a zinc base alloy coating comprising about 1% to about 5% by weight magnesium and from about 3% to about 17% by weight aluminum with the balance being formed essentially of Zinc which can contain the usual minor amount of impurities in a zinc spelter. Whereas it has heretofore been thought that the optimum properties for a zinc base alloy coating having between 1.5 and 4% by weight magnesium and preferably about 3% by weight magnesium could be achieved by combining therewith not more than a fraction of 1% by weight aluminum, it has now been found that the corrosion resistance and other properties of the magnesium-zinc alloy hot dip coatings can be unexpectedly improved by incorporating therein between about 3% and 17% by weight aluminum. It has also been found that protective hot dip coatings within the foregoing composition range exhibit special properties useful for specific applications, as wi l be described hereinafter.

The improved alloy coatings of the present invention when applied to a ferrous metal base exhibit extremely good corrosion resistance. The data in the following Table I illustrates the significant improvement in corrosion resistance of a relatively thin hot dip alloy coating containing magnesium, aluminum and zinc within the range of concentrations specified by the present invention compared with a 3% magnesium-zinc hot dip alloy coating containing 0.2% by weight aluminum and a conventional zinc hot dip coating of substantially the same thickness made with standard zinc spelter on a conventional continuous hot dip galvanizing line:

1 Without failure.

The coating compositions of the present invention which contain a major proportion of the eutectic mixture composition comprising 2.49% by weight magnesium, 4.39% by weight aluminum and the balance zinc and having a melting point of about 640 F. can be applied as a hot dip coating at a temperature below the normal operating temperature of the conventional continuous zinc galvanizing bath or at a bath temperature below about 850 F. Only when the aluminum content of the ternary alloy bath reaches a level of about 8% by weight and above is it necessary to operate the bath at a temperature above 850 F.

Additional savings can be effected in a hot dip coating process of the present invention, because of a reduction in the top skimming losses caused by surface oxidation of the bath, particularly where elevated coating bath temperatures are used. The data of Table II show the results of static skimming tests carried out in a stainless steel pot at an operating temperature of 850 F:

It has also been observed that no significant intermetallic layer is formed at the interface between the alloy coating layer and the ferrous metal base when the hot dip coating bath of the present invention is maintained at any temperature between the melting point of the coating bath and a temperature of 950 F. Thus, the protective alloy coating formed has improved physical properties, including improved drawability and formability, because of the substantial absence of a layer of brittle intermetallic compound.

In order to further illustrate the present invention the following specific examples are given, without, however, limiting the invention to the particular materials, proportions and conditions used.

EXAMPLE 1 A series of 4" x 8" steel test panels were obtained from 2-0-gauge full hard steel sheets which had a thickness of .034 and chemical compositions of about 04% car- 'bon, .29% to .35% manganese, 01% to 011% phosphorus, .019% to 020% sulfur and .04% copper, with the balance essentially iron. All panels were precleaned by oxidizing in a furnace at 1650 F. for 30 seconds, and the oxidized panels were then transferred into the dry box which contained the laboratory galvanizing equipment. The atmosphere inside the dry box contained 10% hydrogen with the balance nitrogen. The dew point inside the dry box was always kept below F. during the dipping operation. The cleaned panel was preheated at 1800 F. for 3 minutes in the reducing atmosphere of the dry box to effect removal of all surface oxides and then cooled to the bath temperature for dipping while maintained within the reducing atmosphere of the dry box.

An alloy comprising about 2.49% by weight magnesium, 4.39% aluminum and the balance being essentially zinc was melted in graphite pot at a temperature of 4 about 750 F. Three test series of panels were coated with the coating bath temperatures held at 750 F., 800 F. and 850 F., respectively. The immersion time for each was maintained at about 5 seconds to provide an average coating weight of .66 0z./ft. These test panels were then formed by punching a 1-inch diameter hole therein and the wall portion around each hole was extruded to provide a tubular section extending /2-inch above the sheet surface. The standard 5% salt spray test performance results of the panels prepared in the foregoing manner are shown in the following Table III:

TABLE III Average Average coating time to weight failure (oz./ft. (red rust) Coating bath composition per side in hours Normal Zinc Speltor (control) 66 380 2.40% Mg plus 4.30% Al plus 03% Zn 66 1 4, 000 2.40% Mg plus 4.39% Al plus 93% Zn 66 1 4, 000

1 Before forming. 1 After forming.

It can be seen from the salt spray test results in Table III that the alloy coating of Example 1 even after forming exhibits about ten times the corrosion resistance of a normal galvanized coating. This alloy coating also exhibited very satisfactory adhesion, good formability, and good painting characteristics.

In addition to the foregoing improved properties of the new ternary alloy coating, there are also important operating advantages achieved when using this alloy in a hot dip coating bath composition. Thus, the objectionable black particles which are formed in a 3% magnesium-0.2% aluminum-zinc coating bath are eliminated and substantially smaller skimming losses are taken, even at an elevated bath temperature. The lower losses at elevated bath temperature make it possible to operate the hot dip alloy coating bath efficiently over a wider temperature range, including operating at the relatively high temperature of 950 R, if considered desirable.

EXAMPLE 2 A series of panels were prepared and coated as in Example 1 by immersing the panels in a hot dip coating bath containing 2% magnesium, 4% aluminum and 94% zinc spelter to to provide an average coating weight of .33 oz./ft. per side. These test panels in a 5% salt spray test were resistant to corrosion failure (red rust) for about 2000 hours, as compared with hours for panels having the same weight of a conventional hot dip zinc coating.

EXAMPLE 3 A series of panels were prepared and coated as in Example 1 by immersing in a hot dip coating bath containing 2.4% magnesium, 3.8% aluminum and 93.8% zinc spelter to provide an average coating weight of .41 oz./ft. per side. The test panels in 5% salt spray test were resistant to corrosion failure (red rust) for 2100 hours, as compared with 228 hours for panels having the same weight of a conventional hot dip coating.

EXAMPLE 4 A series of panels were prepared and coated as in Example 1 by immersing in a hot dip coating bath containing 2.4% magnesium, 3.2% aluminum and 94.4% zinc spelter to provide an average coating weight of .39 oz./ft. per side. The test panels in 5% salt spray test were resistant to corrosion failure (red rust) for about 1600 hours, as compared with 200 hours for panels having the same coating weight of a conventional hot dip zinc coating.

EXAMPLE 5 A series of panels were prepared and coated as in Example 1 immersing in a hot dip coating bath containing 2.5% magnesium, 43% a uminum and about 93% zinc face oxidation until substantially solidified, the hot dip alloy coatings formed on a ferrous metal surface has optimum Salt Fog corrosion resistance and alsoexhibits a brighter, more lustrous appearance than the ternary hot dip alloy coatings containing a smaller percentage of aluminum. The following table summarizes the Salt Fog test data:

TABLE IV.-ACCELERATED CORROSION TEST DATA ON Al-Mg-Zn COATINGS It has been further unexpectedly found, when the aluminum content of the magnesium-aluminum-zinc alloys of the present invention is increased to at least 8% by weight and up to about 17% by weight aluminum, that a much smoother, more uniform hot dip alloy coating substantially free of objectionable surface irregularities, such as pitting, can be formed, particularly when the hot dip coating is applied in the manner described in Example 15, thereby providing a markedly improved appearance as compared with the corresponding zinc-magnesium alloy coatings containing less than 8% by weight aluminum applied in the same manner. The following Table V shows the changes in coating appearance with changes in aluminum content of the alloy coatings:

TABLE VI.HUMIDITY CABINET CORROSION TEST RE- SULTS TEST CONDITIONS, 140 F., 98% RH. [Average for Triplicate 4" x 6, 20 Gauge Panels] Average coating thickness 1 Corrosion I per slide rate in Nominal coating compositions in mils mils/year 1 As indicated by Permascope. 2 Determined by weight loss.

The improved corrosion resistance properties of the ternary magnesium-aluminurn-zinc base alloys of the present invention are further shown by the data of the following Table VII:

TABLE VII.-CORROSION RATES ON ALUMINUM-MA G- NESIUM-ZINC COATINGS AS INDICATED BY IMMERSION TESTS AT 180 F. FOR A 96 HOUR TEST PERIOD IPY* IPY* Coating Composition in tap in 1% H2O N aCl Inches per year.

TABLE V.OPE RATING CONDITIONS, COATING APPEARANCE AND COATING THICKNESS UNIFO RMITY FO R Al-Mg-Zn COATINGS PRODUCED WITH NITROGEN BLOWING Operating conditions Coating thickness in mils on 20 gauge steel Approximate B ath nitrogen N itrogon Top Bottom Relative temp., flow rate, temp, coating Coating Composition F. o.f.m. F. Range Average Range Average appearance Al, Pb, balance Zn (control) 885 90 11s 25 lgjfi g Al, 3% g, balance n 815 94 110 .30 gag-1 2 rig t. 4% A1, 3% Mg, bal ce t 790 8 11 0 jrgitted, 51 rip gi .2 23 uller an 4 6% Al, 3% Mg, balance Zn 83 79 108 D fi Surface? 8% Al, 3% Mg, balance Zn s10 99 100 snlaloth 10% Al, 3% Mg, balance Zn 915 94 110 ggg The hot dip alloy coatings of the present invention containing between 8 and 17 by weight aluminum also have good adherence properties and have an unobjectionable amount of intermetallic compound formed. When the aluminum content of the alloy is greater than 17%, however, there appears to be an abrupt change in the hot dip coating characteristic, and an aluminum-iron alloy layer of considerably increased thickness is formed. The change in the coating characteristic may be due to the change from a coating containing the ternary eutectic mixture composition to a coating comprised of the tat-aluminumzinc solid solution which occurs at about 17.2% by weight aluminum in the aluminum-zinc phase diagram.

Ternary zinc-aluminum-magnesium alloy coating compositions of the present invention containing at least 8% by weight aluminum also exhibit unexpectedly good corrosion resistence in a high humidity atmosphere, as shown by the Humidity Cabinet Test Data in the following Table VI:

now possible to provide a ferrous metal surface, such as a strip of black plate or the like, with an adherent protective coating which has prolonged resistance to corrosion when the coating is applied in accordance with the present invention in conventional coating weights. Furthermore, corrosion resistance equal to present galvanized coatings can be obtained when the ternary alloy of the present invention is applied in a coating weight considerably less than the coating weights used for conventional galvanized coatings, while the resultant savings and other advantages inherent in a very thin protective coating.

It should be understood that the novel alloy coatings of the present invention can be applied to a ferrous metal surface or other corrodible metal surface by means other than by the hot dip coating method specifically used herein to illustrate the present invention, particularly where it is desired to apply very thin alloy coatings of the order spelter to provide an average coating weight of .43 oz./ft. per side. The test panels in 5% salt spray test were resistant to corrosion failure (red rust) for 2800 hours, as compared with 228 hours for panels having the same weight of a conventional hot dip zinc coating.

EXAMPLE 6 A series of panels were prepared and coated as in EX- ample 1 immersing in a hot dip coating bath containing 1.5% magnesium, 4.4% aluminum and about 94% zinc spelter to provide an average coating weight of .41 oz./ft. per side. The test panels in 5% salt spray test are resistant to corrosion failure (red rust).

EXAMPLE 7 A series of panels were prepared and coated as in Example 1 immersing in a hot dip coating bath containing 3% magnesium, 6% aluminum and 91% zinc spelter to provide an average coating weight of .41 oz./ft. per side. The test panels in 5% salt spray test were resistant to corrosion failure (red rust) for over 1000 hours without failure, as compared with 300 hours to failure of panels of a conventional hot dip zinc coating having the same coating weight.

EXAMPLE 8 A series of panels were prepared and coated as in Example l immersing in a hot dip coating bath containing 5% magnesium, 4% aluminum and 91% zinc spe ter to provide an average coating weight of .43 oz./ft. per side. The test panels in 5% salt spray test are highly resistant to corrosion failure (red rust).

EXAMPLE 9 A series of panels were prepared and coated as in Example 1 immersing in a hot dip coating bath containing 3% magnesium, 8% a uminum and 89% zinc spelter to provide an average coating weight of .41 oz./ft. per side. The test panels in 5% salt spray test were resistant to corrosion failure (red rust) for over 1000 hours without failure, as compared with 300 hours to failure of panels of a conventional hot dip zinc coating having the same coating weight.

EXAMPLE 10 A series of panels were prepared and coated as in Example l immersing in a hot dip coating bath containing 3% magnesium, 10% aluminum and 87% zinc spelter to provide an average coating weight of .41 oz./ft. per side. The test panels in 5% salt spray test were resistant to corrosion failure (red rust) for over 1000 hours without failure, as compared with 300 hours to failure of panels of a conventional hot dip zinc coating having the same coating weight.

EXAMPLE 1 l A series of panels were prepared and coated as in Example 1 immersing in a hot dip coating bath containing 5% magnesium 8% aluminum and 87% zinc spelter to provide an average coating weight of .43 oz./ft. per side. The test panels in 5% salt spray test are highly resistant to corrosion failure (red rust).

EXAMPLE 12 A standard steel hot mill band having a thickness of about 0.080 inch is cold reduced on a five-stand tandem mill to form a steel strip having a thickness of about 0.0153 inch thick (Le. 28 U.S.S.G.). The full hard 0.0153 inch strip thus formed having a Rockwell hardness (30 T-scale) of about 80 is cleaned by passing through a continuous cleaning line, followed by conventional box annealing heat treatment to restore the ductility lost when the strip was cold reduced. The annealed strip is then temper rolled to provide a suitable surface for continuous hot dip coating.

The annealed endless steel strip after cleaning as in Example 1 or by other suitable means was passed into a zone having a dry reducing atmosphere of the type used in Example 1 to remove all surface oxidation and whi e in the protective reducing atmosphere is immersed in a molten aluminum-magnesium-zinc alloy bath having the following compositions:

Percent by weight Aluminum 10.00

Magnesium 3.72 Iron 0.06

Lead 0.013

Zinc 86.206

The alloy bath was maintained at a temperature of 900 F. and the steel strip conveyed through the bath at a uniform line speed maintained at about 30 to 40 feet per minute. The strip while enclosed in the protective reducing atmosphere was fed into the alloy bath at an angle of about 70 to the horizontal. The strip was allowed to remain in the alloy bath about 5 seconds (and not more than 10 seconds), and was continuously Withdrawn from the bath at an angle of about to the horizontal. As the alloy coated strip was withdrawn from the bath, the molten alloy coating was rapidly cooled and the coating weight controlled by blowing thereover nitrogen gas having a temperature below F. and preferably at about 50 F. The nitrogen also serves to protect the coating against oxidation until the coating solidifies to form a uniformly smooth surface. An alloy coating was obtained having a substantially uniform thickness on each side of the strip maintained between about 0.2 mil and 0.6 mil, depending on the line speed and the nitrogen blowing conditions maintained. The latter alloy coating exhibited the improved properties of the coating of Example 1.

EXAMPLE 13 A 28 gauge steel strip was prepared and coated as in Example 12, but wherein the ternary alloy hot dip coating bath had a temperature of about 910 F. and contained on a weight basis 12% aluminum, 3% magnesium and the balance zinc. The resultant coating had an average coating weight per side of 0.2 mil and exhibited good corrosion resistance and a smooth surface appearance.

EXAMPLE 14 A 28 gauge steel strip was prepared and coated as in Example 12, but wherein the ternary alloy hot dip coating bath had a temperature of about 940 F. and contained on a weight basis 16% aluminum, 3% magnesium and the balance zinc. The resultant coating had an average coating wcight per side of 0 .2 mil and exhibited good corrosion resistance and a smooth surface appearance.

EXAMPLE 15 A 28 gauge steel strip was prepared and coated as in Example 12, but wherein the ternary alloy hot dip coating bath had a temperature of about 920 F. and contained on a weight basis 13% aluminum, 5% magnesium and the balance zinc. The resultant coating had an average coating weight per side of 0.2 mil and exhibited good corrosion resistance and a smooth surface appearance.

EXAMPLE 16 A 28 gauge steel strip was prepared and coated as in Example 12, but wherein the ternary alloy hot dip coating bath had a temperature of about 950 F. and contained on a weight basis 17% aluminum, 5% magnesium and the balance zinc. The resultant coating had an average coating weight per side of 0.2 mil and exhibited good corrosion resistance and a smooth surface appearance.

It has been unexpectedly found that when the ternary magnesium-aluminum-zinc base alloy compositions of the present invention containing about 3 to 4% by weight aluminum and about 3% by weight magnesium which corresponds to about the ternary alloy euthetic point are applied to a ferrous metal surface free of oxides and preferably where the alloy coating is also protected against surof about 0.1 oz./ft. For example, a thin alloy coating can be applied by the vapor deposition technique, wherein a strip of steel is continuously passed through a chamber substantially free of oxygen having the interior at a pressure of about 10" mm. Hg and providing therein adjacent the surface of the moving strip a source of the coating alloy in gaseous form which condenses on the surface of the moving strip. Very thin alloy coatings of the present invention can also be formed by using special wiping techniques on the strip emerging from a hot dip coating bath. The alloy coating of the present invention can also be applied by powdered metallurgy techniques, wherein a powdered alloy having the herein specified proportion of magnesium, aluminum and zinc is formed into a suitable paste and a thin layer is applied which is then cold reduced, sintered, rolled, and heat treated in accordance with conventional practice.

It should also be understood that whereas the foregoing specific examples have used zinc spelter which contain the usual impurities associated with commercial grade zinc, it is possible, but not necessary, to use substantially pure zinc in the same proportion to prepare the alloy formulations of the specific examples. Thus, the term zinc as used in the claims is intended to cover both substantially pure zinc and zinc spelter or other commercial forms of zinc.

We claim:

1. A ferrous metal sheet having on at least one surface thereof an adherent smooth zinc base alloy coating comprised essentially on a weight basis of between about 1% and 5% magnesium and between about 3% and 17% aluminum with the balance being essentially zinc.

2. A ferrous metal sheet as in claim 1, wherein said coating is an adherent smooth hot dip zinc base alloy coating comprised essentially on a weight basis of between about 1% and 5% magnesium and between about 6% and 17% aluminum with the balance being essentially zinc.

3. A ferrous metal sheet as in claim 1, wherein the said coating is comprised essentially on a weight basis of about 1% to 5% magnesium and between about 3% and 4% aluminum with the balance being essentially zinc.

4. A ferrous metal sheet as in claim 1, wherein the said coating is comprised essentially on a weight basis of between about 3% and 5% magnesium and between about 8% and 17% aluminum with the balance being essentially zinc.

5. A ferrous metal sheet as in claim 1, wherein said coating is comprised essentially on a weight basis of about 3% magnesium and about 4% aluminum with the balance being essentially zinc.

6. A ferrous metal sheet as in claim "1, wherein said coating is comprised essentially on a weight basis of about 4% magnesium and about 10% aluminum with the balance being essentially zinc.

7. A ferrous metal sheet as in claim 1, wherein said coating is a hot dip coating comprised essentially on a weight basis of between about 3% and 5% magnesium and between about 8% and 17% alumnium with the balance being esentially zinc.

References Cited UNITED STATES PATENTS 2,703,766 3/ 1955 Ellis et a1. 3,164,464 1/ 1965 Heath. 3,245,765 4/1966 Lawson. 3,320,040 5/1967 Roe et al.

OTHER REFERENCES Galvanized Panels Improved by Magnesium Additions. *In The Iron Age, 187(23): pp. -102, June 8, 1961, 1 17-1 14 A) ALFRED L. LEAVITI, Primary Examiner J. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

333? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Inventor(s) Lee and Halley ppears in the above-identified patent Patent are hereby corrected as shown below:

3, line 75, "in graphite" should read in a graphite-; 1 4, line 45, "to to" should read -to-; C01. 7, TABLE V, "25" should read ---.25-; C01. 8, TABLE VI, "slide" should read --side-; Col. 8, TABLE VII, "0005" should read .0005-- SKSI'IED MD SEALED (SEAL) Afloat;

. mm 3' JR- Edward M. 11%,, Oomissioner of Patents A mia; ficm- 

